# How Mette Gaarde uses light to measure ultrafast electrons

How do you measure things that move really fast? With light, of course! Dr. Mette Gaarde, Professor of Physics in the Department of Physics and Astronomy, explains how ultrafast pulses of light—think a billionth of a billionth of a second—can be used to uncover very fast processes. We discuss how attosecond light pulses can be applied to learn about electron movement in reactions along with what inspired her to become a physicist. (Transcript below)

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## Transcript

Becky Carmichael

[0:01] This is Experimental, where we explore exciting research occurring at Louisiana State University and learn about the individuals posing the questions. I'm Becky Carmichael, and today we are joined by Dr. Mette Gaarde from the Department of Physics who explains how ultrafast light can be used to uncover very fast processes.

Mette Gaarde

[0:20] Have you ever thought about what it takes to make a movie? Not organizing the actors or designing the sets, but how the live action is actually captured. A movie is generally a series of still pictures put together to show you how some event is unfolding in time. For example, so you can see how the ball was passed from one player to another. Each still picture needs to freeze the action, otherwise it will be blurry. This means that the time to take the picture must be much shorter than the time it takes for your object to move. For instance, if you want to freeze the wings of a hummingbird, which beat really fast, like 50 times per second, there are two ways to do this. You can use a camera with a really fast shutter mechanism. Or you can put the hummingbird in a dark room and only shine light on it for a really short time, like with a big flash, then your camera shadow can just be opened the whole time. My research deals with how to produce and apply very, very short bursts of light that can be used to make movies of things that move very fast. We're interested in the motion of electrons inside atoms, molecules and solids. And these electrons moves so fast that in order to freeze them, we need light pulses that only last a few 10s of attoseconds. And I'm talking fast. One attosecond is a billionth of a billionth of a second. For example, attosecond light pulses can help us watch the earliest steps in a chemical reaction. And once we understand how those first steps take place, maybe we can start to manipulate or control them. Or we could use attosecond light pulses to learn about how the light harvesting process of a biomolecule works. Biomolecules that absorb and convert UV light as part of the photosynthesis process. Attosecond light pulses are made in laser laboratories in the US and around the world. My group is a theory group so we do calculations to support and make predictions for these experiments. For example, my Experimental collaborators may contact me about a measurement that shows the attosecond light pulses behaving in some particular way. Then my job would be to help them explain why that is. And maybe in the process of explaining the behavior, we come up with new experimental tests that they could do in the lab. One of the most fascinating projects I have been involved in recently is about the generation of these light pulses in transparent crystals. Until just a few years ago, this was thought to be impossible. It was believed that the high density of the material in the solid state would make it too difficult for the light to make it out of the crystal. However, recent experiments in theory have shown us that it is not just possible, but that these light pulses can teach us a lot about how to rapidly control electric currents in the solid state of matter. This is important because all electronic devices rely on billions of microscopic currents being switched on and off every second. So if attosecond science can help us control these currents, it may help us to dramatically increase the speed of electronic signal processing.

Becky Carmichael

[3:34] So, hi. My name is Becky Carmichael, and I'd like to welcome Mette Gaarde today to our show. So Dr. Gaarde, would you tell me a little bit about who you are and what do you do here at LSU?

Mette Gaarde

[3:44] Right, so I am a professor in the Department of Physics and Astronomy, and I am a theoretical physicist. As you have heard in my monologue, my research deals with ultra fast processes. We are interested in things that happen on the shortest timescales. And I do calculations that support experiments that deal with this kind of physics.

Becky Carmichael

[4:08] So I was pretty impressed when you said the attosecond and that it's a billionth of a billionth of a second. What drew you to this particular area physics,

Mette Gaarde

Becky Carmichael

[6:37] From those initial experiences, kind of that brought you into this area of research. I also like that you're talking about how the methods of teaching that these professors that you've encountered would use to demonstrate these processes? Do you use any of those in your own classroom or for your own graduate students that you'd like like to admit?

Mette Gaarde

[7:02] I do not believe I run around tables. No, I have not quite gone there. But I'm certainly very aware of that physics needs to be conveyed with enthusiasm and with examples, and in that the more concrete you can make things when you explain them, the easier it goes over with students. So...

Becky Carmichael

[7:25] And the other piece that you had mentioned is the collaboration. So are there any interesting collaborations that you've been involved with, or you're currently involved with that help further the work that you're doing in your own lab?

Mette Gaarde

[7:40] Right. So I would say I have been tremendously lucky in being a theoretical physicist and being allowed to work so closely with actually a number of different experimental groups, which is just very enriching. When you can see that what I do, which generally involves a computer and sitting in front of my computer, and talking to my theory collaborators, and then that you see how this really works. I mean, it really works in the lab, or you can help explain some experimental result and this back and forth that we have between theory and experiment and to be involved so closely in that. I have been very grateful to be involved in that.

Becky Carmichael

[8:26] So aside from, you know, having a solid understanding and keeping engaged with the research happening with this area physics. Are there other skill sets that you find that you've needed and you've employed during your duration as a faculty member, as a graduate student, as a researcher?

Mette Gaarde

[8:46] If you're asking sort of about what would I tell a student who wanted to have a career in physics, for example, I would say it's really important to dare to ask. To stay curious. To not just take the answer for granted that you're given. To go further. And sort of for a student in the classroom, I would also say don't be afraid to not know the answer. I mean, most of the time, when you're a researcher, you don't know the answer. There's a problem. And you know maybe how to get to the answer, but you certainly don't know the answer when you start. And so don't be afraid to not know. It's okay to ask.

Becky Carmichael

[9:31] You mentioned a little bit about an organization that you're involved with for women in physics. Can you tell us a little bit about that?

Mette Gaarde

Becky Carmichael

[11:29] And so if someone's interested in becoming part of this organization, is there a local chapter here at LSU?

Mette Gaarde

[11:36] So um, the conferences are primarily for physics majors or for students who might have an interest in undertaking a physics major or a physics minor. They can contact me if they're interested in attending next year, for instance. We would be happy to have them. And there's also... Generally there is a society of physics students who would be another another place to contact.

Becky Carmichael

[12:01] I want to step back to you for a minute, and I want you to think about yourself when you were younger. Is there one thing if you could go back in time and tell your younger self, what would it be?

Mette Gaarde

[12:12] Not to worry so much. You know, there are many different ways that things can go right. And it doesn't all have to be... You can't imagine what's going to happen. And so when opportunities arise, grab them. Just say, yes. Try it out. I would say, yeah. Don't be afraid to be wrong. Don't be afraid to fail. You should try things, and it might go better than you think.

Becky Carmichael

[12:42] I think that's also great advice. I think that there's some things that you learn, even if it is... If it fails, it's okay. I think there's still things to grow from. And so I think... I always liked this idea of grabbing opportunities, you never know how that's going to change and fluctuate, or develop and grow that career path. That's wonderful. Well, has there been something that I haven't asked you that you've wanted to share kind of with students or with the community? Is there something that we didn't really get to touch on yet?

Mette Gaarde

[13:12] We have talked about it a little bit. I mean, I think that physics is exciting. I think that I wish more students would would share that excitement. I think a lot of students think of physics is hard or oh, I need a lot of math to be a good physicist. And eventually you do need to learn math to be a good physicist, but you can certainly be excited by physics and be engaged in physics and be curious about physics and before you have mastered a lot of complicated math. And I wish more students would share our excitement about that, excitement about asking questions about the natural world and how things work. That's what we work on.

Becky Carmichael

[13:55] Is there something or an example that you tend to lean on when you're trying to share that enthusiasm with the students? Kind of your go to example. I know you talked about, you know, to the ball, how did the ball move from point A to point B? But is there some kind of example that you use that is very relevant for daily life? I totally put you on the spot.

Mette Gaarde

[14:17] Yeah.

No. I don't think I have anything right out of the hat. No, I mean, let's, again, let's move on.

Becky Carmichael

[14:23] I know. I'm sorry.

Move on from that one? Okay. I think I've asked you pretty much all the questions that I had.

Mette Gaarde

[14:36] I mean, so what were some of the ones we skipped? One of them was "what do you see yourself doing in the future?" I think this one is hard in the sense that, you know, research is hard to predict, right? I mean, what am I doing if I'm doing research in 10 years from now? That's very hard to say what... Where it's going. In five years, maybe, we are thinking about... I can sort of see the horizon for where the projects are going. 10 years? This is hard.

Becky Carmichael

[15:06] So then in five years, do you think... How do you see that your work is going to contribute to that, that greater unknown? What is the piece that you see yourselves to be uncovering?

Mette Gaarde

[15:16] Right, so I think that the last thing that I talked about in my monologue about how to make attosecond light pulses in transparent crystals, I think this project, not necessarily just my part of it, but I think the community is making huge steps in that direction. How to control microscopic currents. How to understand microscopic currents. And I think that the effort that's being done there in terms of sort of controlling currents with light, controlling currents that could influence macroscopic processes, I mean, inside of electronic devices, I think that could really be going somewhere. Not necessarily just from my group, but from sort of a worldwide effort, and I think that's very exciting to think about.

Becky Carmichael

[16:06] I think that that does sound very exciting. So I guess kind of going back to the idea of those early stages of a reaction. I think that's very interesting in terms of talking about kind of the energy that's transferred. Where's that energy being... How can we harness that energy, maybe make it more efficient? Do you see that that's something too?

Mette Gaarde

[16:27] I think, in general, all of this understanding leads to control. That's a very general idea in my field that if you can map out how something happens, that's the first step in learning how you can change how something happens. And change is then a very general way of saying, Yes, I do not think that attosecond science is going to change how bacteria perform photosynthesis. But I think maybe something much more modest, that you can can change some fundamental step in some in some reaction. Again, can you use light to control matter? That is really the fundamental question that we're asking.

Experimental Podcast

[17:17] Experimental was recorded and produced in the KLSU Studios here on the campus of Louisiana State University and is supported by LSU's Communication Across the Curriculum and the College of Science. Today's interview was conducted by Becky Carmichael and edited by Bailey Wilder. To learn more about today's episode, subscribe to the podcast, ask questions, and recommend future investigators visit cxc.lsu.edu/experimental